Jet propulsion nozzle

ABSTRACT

A variable area and reversing propulsion nozzle arrangement includes first and second sets of opposed valves or &#39;&#39;&#39;&#39;eyelids&#39;&#39;&#39;&#39; rotatable towards each other to reduce the area of the outlet of a jet pipe, the two sets of valves being rotatable about crossed axes. Reverse thrust outlets are disposed in the wall of the jet pipe between the axes of valve rotation, these including rotatable vane cascades which may close the outlets or direct the flow forwardly. The thrust reversing cascades and the area varying valves are coupled to common actuators, the arrangement being such that the reversing cascades begin to open only after the propulsion nozzle is partly closed.

United States Patent mal Sept. 12, 1972 [54] JET PROPULSION NOZZLE 3,567,128 3/1971 Urquhart et al. .....239/265.37 [72] Inventor: Charles H. Sma1e,lndianapolis,lnd.

Primary Examiner-Lloyd L. King 1 Asslsnw General Motors Corporation, Attorney-Paul Fitzpatrick et a1.

Detroit, Mich.

221 Filed: June 23, 1971 1 1 ABSTRACT 2 1 App] 155,32 A variable area and reversing propulsion nozzle arrangement includes first and second sets of opposed valves or "eyelids" rotatable towards each other to [52] U.S. Cl ..239/265.29, 239/265.37 reduce the area of the outlet of a jet pipe, the two sets [51] lllLCl "B640 15/04 of valves being rotatable about crossed axes Reverse [58] Field of Search ..239l265.l9, 265.37, 265.39, thrust outlets are disposed in the W8" of the jet pipe 239/2654! between the axes of valve rotation, these including rotatable vane cascades which may close the outlets or [56] References cited direct the flow forwardly. The thrust reversing UNITED STATES PATENTS cascades and the area varying valves are coupled to common actuators, the arrangement being such that 2,735,264 2/1956 Jewett ..239l265.;37 the reversing cascades begin to open only after the 3,002,342 10/1961 Schatzkr ..239/265.37 propulsion nozzle is partly c|ose 3,512,716. 5/1970 Kopp ..239/265.29 3,531,049 9/1970 Hom ..239/265.39

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UmdasHSmale PAIENTEDsEr 12 m2 SHEET [If 4 AIIUHLJH V ET PROPULSION NOZZLE DESCRIPTION My invention relates to variable propulsion nozzles for jet propulsion engines such, for example, as turbojet engines. It involves arrangements for varying the area of the nozzle to accommodate to different conditions of engine operation and also means to block the normal propulsive outlet and direct the flow through a reverse thrust outlet for braking the vehicle.

In the preferred embodiment of the invention, the propulsive outlet is variable from a maximum area suitable for afterburning operation of a jet engine to a reduced area suitable for non-afterbuming operation and still further to a position closing the normal outlet so as to permit operation of the reverse thrust portion of the device. A feature of the invention is the use of two sets of valves or eyelids arranged in quadrature so that the jet propulsion outlet in both afterbuming and normal operation is a square with rounded convex sides reasonably approximating the ideal circular shape.

The principal objects of my invention are to improve the operation of jet propulsion engines, to provide an improved variable area and thrust reversing nozzle arrangement for jet propulsion engines, and to provide improved structural and actuating arrangements for variable propulsion nozzles. A still further object of the invention is to provide an improved combination of a normal jet propulsion outlet blocking means and a reverse thrust nozzle activating means.

By way of introduction to the invention, it may be pointed out that various arrangements for blocking the main outlet of an engine and opening a thrust reverser have been proposed,-two of which are disclosed in Friedmann US. Pat. No. 2,994,193 of Aug. 1, 1961 and Mattia et al. US. Pat. No. 3,344,604, Oct. 3, 1967. Also, variable-area jet nozzles for afterburning and other jet engines have been common for many years. However, I believe that my invention in its preferred embodiment provides a very elegant and practical solution to the problem of flexibility in operation of a jet engine exhaust system.

The nature of my invention and its advantages will be clear to those skilled in the art from the succeeding detailed description of the preferred embodiment of the invention.

Referring to the drawings,

FIG. 1 is an axonometric view of the tail end of an aircraft illustrating the exhaust nozzle arrangement.

FIG. 2 diagrammatically illustrates the full open or afterbuming position of the nozzle.

FIG. 3 diagrammatically illustrates the closed or thrust reversing position of the nozzle.

FIG. 4 is a partial transverse sectional view of the jet nozzle taken on the plane indicated by the line 4-4 in FIG. 1, looking downstream.

FIG. 5 is a partial longitudinal sectional view of the same taken on the broken plane indicated by the line 5-5 in FIG. 6.

FIG. 6 is an elevation view of a thrust reversing cascade.

FIG. 7 is a longitudinal sectional view of the nozzle, with parts cut away and in section.

FIG. 8 is a sectional view of a nozzle valve pivot arrangement taken on the plane indicated by the line 8- 8 in FIG. 7.

FIG. 9 is a longitudinal sectional view of the nozzle taken on a plane at right angles to that of FIG. 7.

FIG. 10 is a partial view illustrating a nozzle actuator.

Referring first to FIG. 1, an installation in the tail of an aircraft is shown. The aircraft includes a fuselage 2 bearing horizontal stabilizers 3 and a vertical stabilizer 4, with the usual movable control surfaces. The fuselage terminates in a rear wall 6 from which projects the downstream or outlet end of a jet pipe or tailpipe 7. The tailpipe conveys the motive fluid delivered by a jet propulsion engine (not illustrated). Such an engine may be of any suitable type. In the installation described, it includes an afterbumer or reheat burner in the tailpipe. The tailpipe is of circular cross section and roughly cylindrical, but with some variations in diameter. The tailpipe terminates in a circular outlet 8. The terminal portion of the tailpipe is approximately in the form of a spherical zone 10 to house the nozzle area varying valves. There is a pair of outer valves 11 (see also FIGS. 2, 3, and 9) of a type known in the industry as eyelids reversely rotatable about an axis 12. There is a second or inner pair of valves 14 reversely rotatable about an axis 15 preferably normal to the axis 12, both the axes being normal to the axis of the tailpipe.

Each of the valves is substantially in the form of a lune of a sphere, the outer valves 11 being of slightly greater diameter than the inner valves 14 'so that they may move relative to each other. The two valves of each set are mounted on concentric pivots at each end of the axis to be described more fully, and each pivot is rigid with an arm 16 exterior to the jet pipe but within the fuselage, the arms 16 being coupled by links 18 to the reciprocating member 19 of a hydraulic actuator cylinder or other suitable reciprocable actuator 20. There are, thus, in the preferred installation four actuators aligned with the axes l2 and 15. It will be obvious from the geometry of the linkage from the actuators to the valves that they move oppositely but to. equal extents as also illustrated generally in FIGS 2, 3, 7, and 9. This general type of operating linkage for the valves is known as shown, for example, by US. Pat. No. 2,704,436 of Short et al., Mar. 22, 1955. These actuators may be synchronized by any suitable means, the art of synchronized hydraulic actuators being very well developed. However, mutual synchronication of two actuators coupled to the same pair of valves is not necessary. Any suitable feedback may be employed, as is well known in the art, so that the valves may be moved to any desired position or may be held in any particular desired position. If it is deemed sufficient to have three positions only of the valves, a wide-open position for afterbuming operation, a partially closed position for dry or non-afterburning engine operation, and a closed position for reverse thrust for aircraft braking, known three-position actuator cylinders may be used. However, modulation of the position of the valves to provide an infinitesimally variable area is easily achieved, if desired.

Considering now in more detail the specific arrangement of the variable-area propulsion nozzle, we may refer particularly to FIGS. 7, 8, 9, and 10. The tailpipe 7 is a double-walled structure with an outer wall 22 and an inner wall 23. These are joined near the downstream end of the inner wall by spacers 24 fixed to the walls. The terminal portion 26 of the jet pipe, which is singlewalled, is bolted to wall 22 at a flange 27. An annular sealing member 28 is also fixed to these at the flange 27. The downstream end of inner wall 23 is a flaring portion 29 in the form of a spherical zone which constitutes part of the generally spherical zone shaped downstream end of the tailpipe. The inner valves 14 in their fully retracted or wide open position illustrated in solid lines in FIG. 7 fit over the portion 30 of the inner wall and within the seal 28 with a view to minimizing leakage into the space between walls 23 and 22 of the tailpipe. In the specific device illustrated, the leading edge of each inner valve is in a plane about fifty-two degrees around the axis from the plane of the trailing edge of the valve.

The outer valves 11 (FIG. 9) are of slightly greater diameter than the inner valves 14 so that they are received over the outer surface of valves 14 in the terminal portion 26 of the exhaust duct. These valves are of less angular extent, about 28, than the inner doors and do not enter the space between the flare 29 and seal 28. The open position of these valves is shown in solid lines in FIG. 9, and their closed position in broken lines.

Each valve of each set is fixed to two rotatable hubs, one at each end of the axis 12 or 15, forming part of a concentric hub arrangement 30. FIG. 8 illustrates particularly the hub arrangementof the inner valves 14. One valve 14 is bolted or otherwise fixed to a flange 31 forming part of a hub assembly 32. The hub assembly includes a ring 34 bearing flange 31 and a ring 35, these being riveted or otherwise fixed together at flanges 36. The hub assembly 32 is rotatably mounted in the cylindrical interior of a boss 38 extending outward from the tailpipe wall 26, radial bearings being provided by bushings 39,, one mounted on each of rings 34 and 35. An axial locating bearing is provided by a flange 40 on the inner surface of the boss 38 which lies between flanges on the rings 34 and 35. One of the'actuating arms 16 is welded or otherwise fixed to the ring 35.

The other valve 14is bolted or otherwise fixed to a flange 42 extending from an inner hub 43 rotatably supported in the, hub assembly 32. The support is provided by a bushing 44 mounted on the inner end of hub 43 and a bushing 46 mounted in the interior of ring 35. The operating arm 16 for the inner hub 43 is. integral with a bolt 47 whichhas an enlarged head 48. Head 48 bears against an internal flange on ring 35 to locate the inner hub in one axial direction of movement. A flange 49 on the inner hub which abuts the flange 36 on ring 34 locates in the opposite axial direction. Bolt 47 is threaded to receive a nut 50 protected by a cap 51, the nut bearing against an internal flange on the inner hub 43. Bolt 47 is nonrotatably coupled to hub 43 by suitable means such as a key 52. The hub arrangements for the outer valves 1 l are essentially the same as that illustrated in FIG. 8.

When the valves 14 are moved forwardly, they lie inside the hubs for the valves 11. The valves 11 in their open position are downstream of the hubs for valves 14, as illustrated in FIG. 9.

The arrangement of the actuator is further illustrated in FIG. 10. The actuator, which as illustrated is a hydraulic cylinder, has a piston rod 19, the outer end of which bears a roller 52 which traverses a channel section guide or track 55. The track 55 is fixed to the bolting flanges 27. The cylinder includes a boss which is coupled by a ball joint to a bracket 56 fixed to the wall 22 of the tailpipe. The links 18 are pivoted by pins 58 to the rod 19 adjacent the location of roller 52.

The reverse thrust arrangement is illustrated particularly in FIGS. 1, 4, 5, and 6. As illustrated in FIG. 4, the aircraft fuselage outer skin 2 is formed with openings 62 at each side of the hub arrangements 30 through which the engine exhaust is directed for reverse thrust. The outer wall portion 26 of the jet pipe is formed to provide rectangular chutes or outlets 63 located within the openings 62. Each chute is bounded by a front wall 64, a rear wall 66, and side walls 67. As will be seen, the front and rear walls incline somewhat forwardly of the aircraft in the radially outward direction. In the preferred embodiment, there are three large size reversing cascades, one at each side and one under the fuselage, and two smaller ones at the top which in effect amount to splitting a large cascade to avoid inter ference with the aircraft rudder. Depending upon the installation, the arrangement of reverser cascades could be varied to suit.

FIGS. 5 and 6 are viewsof one of the wider reversing cascades which may be considered typical; specifically they may be considered to show the side cascade visible in FIGS. 1 and 4. The rectangularopening 63 is filled with a cascade of cambered gas-directing vanes which, in the position illustrated in solid lines, abut each other and abut the forward and rear walls of the outlet 63 to close the outlet. The vanes are coupled by linkage so as to be rotated in unison from this position to the position illustrated in broken lines in FIG. 5 in which they direct the engine exhaust, which cannot flow through the closed normal propulsion nozzle, with a forward component of velocity with respect to the aircraft for reverse thrust. Each vane 68 includes a hub 70 rotatable in one side wall 67 and a shaft 71 extending through the other side wall to which is coupled an actuating arm 72. The other ends of the arms 72 are pivoted to a connecting rod 74 which is reciprocated to rotate the vanes in unison. One end of connecting rod 74 is pivoted at 75 to a rocker arm 76 fixed to a shaft 77, the shaft having ends 78 mounted in brackets 79 on thewall 26. A second rocker arm 80 fixed to the shaft 77 is coupled by an extensible link 82 to the pivot pin 83 which joins it to the link 18 and arm 16 at their common junction. A tension spring 84 coupled between the arm 80 and a bracket 86 on the jet pipe holds theconnecting rod 74 in the direction to close the vane cascade.

The link 82 includes an outer shell 87 which is coupled to the rocker arm 80 and a rod 88 slidably mounted within the shell which is fixed to the end fitfing 90 through which the pivot pin 83 extends. A flange 91 on the rod 88 engages the abutment 92 near the end of shell 87 when the rod 88 is sufficiently extended. The result of this lost motion linkage is that, as the propulsion nozzle valves move from wide open towards closed position, the vane cascade stays closed until the closing movement goes beyond that used for normal propulsion of the aircraft, at which point the flange 91 engages the abutment 92 providing a solid connection from the actuator cylinder 90 to the.vane cascade which is opened against the tension of the spring. When the main nozzle valves are reopened, the spring causes the cascade to close as the valves are opened towards the normal thrust position and allows overtravel of the valve with respect to the turning vanes for the afterbuming position.

In normal operation of the engine, the gas pressure within the exhaust pipe tends to hold the turning vanes 68 closed. It may also be noted that the turning vanes, except for the short ones on the upper side of the jet pipe, are bowed or arcuate as shown clearly in FIG. 4 so as to clear the valves 14 when these are moved to open position.

It will be seen from the foregoing that I have provided an exhaust system for an aircraft engine which permits modulation of the area for normal thrust operation of the engine and for afterburning or for thrust spoiling, and which also provides a simple and effective arrangement for blocking the normal exhaust and operating the thrust reversing mechanism. One advantage of the structure is the more nea'rly circular shape of the outlet as compared to the conventional two-valve variable exhaust nozzle and another is the division of load between four actuators rather than two and the reduction of unit loads on the jet pipe by the use of four rather than two valves to modulate and obstruct the jet nozzle.

The detailed description of the preferred embodiment of the invention for the purpose of explaining the principles thereof is not to be considered as limiting or restricting the invention, since many modifications may be made by the exercise of skill in the art.

I claim:

1. A variable area and reversing jet propulsion nozzle comprising, in combination, a jet pipe of substantially circular cross section terminating in a propulsive outlet; means defining two crossed axes of rotation extending across the jet pipe adjacent to the outlet; two pairs of nozzle area varying valves of spherical contour, one pair being supported in the jet pipe for rotation about each of the said axes to provide a variable jet orifice and to close the orifice; rotating means extending from the jet pipe at the axes coupled to the valves; and actuating means coupled to the rotating means so as to actuate the valves concurrently and the valves of each pair oppositely. i

2. A variable area and reversing jet propulsion nozzle comprising, in combination, a jet pipe of substantially circular cross section terminating in a propulsive outlet; means defining two crossed axes of rotation extending across the jet pipe adjacent to the outlet; two pairs of nozzle area varying valves of spherical contour, one pair being supported in the jet pipe for rotation about each of the said axes to provide a variable jet orifice and to close the orifice; rotating means extending from the jet pipe at the axes coupled to the valves; actuating means coupled to the rotating means so as to actuate the valves concurrently and the valves of each pair oppositely; means defining reverse thrust outlets through the jet pipe between the axes; a cascade of vanes in each reverse thrust outlet for directing gas issuing from the outlet with an upstream velocity component relative to the jet pipe; and means coupling each reverse outlet means to a said actuating means effective to open the outlet means during closing movement of the valves and only after partial closure of the valves.

3. A variable area and reversing jet propulsion nozzle comprising, in combination, a jet pipe of substantially circular cross section terminating in a propulsive outlet; means defining two crossed axes of rotation extending across the jet pipe adjacent to the outlet; two pairs of nozzle area varying valves of spherical contour, one pair being supported in the jet pipe for rotation about each of the said axes to provide a variable jet orifice and to close the orifice; rotating means extending from the jet pipe at the axes coupled to the valves; actuating means coupled to the rotating means so as to actuate the valves concurrently and the valves of each pair oppositely; means defining reverse thrust outlets through the jet pipe between the axes; a cascade of rotatably mounted vanes in each reverse thrust outlet rotatable between a position closing the outlet and a position directing gas issuing from the outlet with an upstream velocity component relative to 'the jet pipe; and vane cascade operating means coupling each cascade to a said actuating means effective to open the cascade during closing movement of the valves only after partial closure of the valves.

4. An area varying jet nozzle for a jet propulsion engine comprising, in combination, a wall of generally circular cross section defining a gas duct, the wall terminating at a propulsive jet outlet; a first pair of opposed valves rotatably mounted to swing oppositely about a first axis transverse to the duct; a second pair of opposed valves rotatably mounted to swing oppositely about a second axis transverse to the two aforementioned axes, the first and second axes being substantially coplanar and being upstream of the jet outlet; all the said valves being substantially in the form of lunes of a sphere of a size such as to substantially fit the wall when rotated to an open position maximizing the jet outlet area defined between the downstream edges of the four valves; and means for moving the valves concurrently to reduce the area of the outlet area from the open area.

5. An area varying jet nozzle for a jet propulsion engine comprising, in combination, a wall of generally circular cross section defining a gas duct, the wall terminating at a propulsive jet outlet; a first pair of opposed valves rotatably mounted to swing oppositely about a first axis transverse to the duct; a second pair of opposed valves rotatably mounted to swing oppositely about a second axis transverse to the two aforementioned axes, the first and second axes being substantially coplanar and being upstream of the jet outlet; all the said valves being substantially in the form of lunes of a sphere of a size such as to substantially fit the wall when rotated to an open position maximizing the jet outlet area defined between the downstream edges of the four valves; means for moving the valves concurrently to reduce the area of the outlet area from the open area to a limit wherein opposed valves meet to close the jet outlet; and reverse thrust outlet means coupled to the valve moving means so as to be made effective upon closing the jet outlet.

6. A jet propulsion nozzle comprising, in combination, a jet pipe having a jet outlet, the portion of the pipe adjacent to the outlet having an interior configuration substantially corresponding to a zone of a sphere; a first set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a first axis transverse to the outlet and diametral to the jet pipe spherical zone, the valves being of a diameter to provide a minimal radial gap between the jet wall and valves; a second set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a second axis transverse to the outlet and the first axis and intersecting the first axis, the valves of the second set being of a diameter to provide a minimal radial gap between the valves of the two sets; and means for rotating the valves about their respective axes concurrently between an open outlet position in which the valves are principally disposed within the said spherical zone and a closed outlet position in which the valves are substantially projected from the spherical zone.

7. A jet propulsion nozzle comprising, in combination, a jet pipe having a jet outlet, the portion of the pipe adjacent to the outlet having an interior configuration substantially corresponding to a zone of a sphere; a first set of opposed valves each substantially in the form of a spherical lune mounted for concurrent'opposite rotation about a first axis transverse to the outlet and diametral to the jet pipe spherical zone, the valves being of a diameter to provide a minimal radial gap between the jet wall and valves; a second set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a second axis transverse to the outlet and the first axis and intersecting the first axis, the valves of the second set being of a diameter to provide a minimal radial gap between the valves of the two sets; means for rotating the valves about their respective axes concur- 

1. A variable area and reversing jet propulsion nozzle comprising, in combination, a jet pipe of substantially circular cross section terminating in a propulsive outlet; means defining two crossed axes of rotation extending across the jet pipe adjacent to the outlet; two pairs of nozzle area varying valves of spherical contour, one pair being supported in the jet pipe for rotation about each of the said axes to provide a variable jet orifice and to close the orifice; rotating means extending from the jet pipe at the axes coupled to the valves; and actuating means coupled to the rotating means so as to actuate the valves concurrently and the valves of each pair oppositely.
 2. A variable area and reversing jet propulsion nozzle comprising, in combination, a jet pipe of substantially circular cross section terminating in a propulsive outlet; means defining two crossed axes of rotation extending across the jet pipe adjacent to the outlet; two pairs of nozzle area varying valves of spherical contour, one pair being supported in the jet pipe for rotation about each of the said axes to provide a variable jet orifice and to close the orifice; rotating means extending from the jet pipe at the axes coupled to the valves; actuating means coupled to the rotating means so as to actuate the valves concurrently and the valves of each pair oppositely; means defining reverse thrust outlets through the jet pipe between the axes; a cascade of vanes in each reverse thrust outlet for directing gas issuing from the outlet with an upstream velocity component relative to the jet pipe; and means coupling each reverse outlet means to a said actuating means effective to open the outlet means during closing movement of the valves and only after partial closure of the valves.
 3. A variable area and reversing jet propulsion nozzle comprising, in combination, a jet pipe of substantially circular cross section terminating in a propulsive outlet; means defining two crossed axes of rotation extending across the jet pipe adjacent to the outlet; two pairs of nozzle area varying valves of spherical contour, one pair being supported in the jet pipe for rotation about each of the said axes to provide a variable jet orifice and to close the orifice; rotating means extending from the jet pipe at the axes coupled to the valves; actuating means coupled to the rotating means so as to actuate the valves concurrently and the valves of each pair oppositely; means defining reverse thrust outlets through the jet pipe between the axes; a cascade of rotatably mounted vanes in each reverse thrust outlet rotatable between a position closing the outlet and a position directing gas issuing from the outlet with an upstream velocity component relative to the jet pipe; and vane cascade operating means coupling each cascade to a said actuating means effective to open the cascade during closing movement of the valves only after partial closure of the valves.
 4. An area varying jet nozzle for a jet propulsion engine comprising, in combination, a wall of generally circular cross section defining a gas duct, the wall terminating at a propulsive jet outlet; a first pair of opposed valves rotatably mounted to swing oppositely about a first axis transverse to the duct; a second pair of opposed valves rotatably mounted to swing oppositely about a second axis transverse to the two aforementioned axes, the first and second axes being substantially coplanar and being upstream of the jet outlet; all the said valves being substantially in the form of lunes of a sphere of a sizE such as to substantially fit the wall when rotated to an open position maximizing the jet outlet area defined between the downstream edges of the four valves; and means for moving the valves concurrently to reduce the area of the outlet area from the open area.
 5. An area varying jet nozzle for a jet propulsion engine comprising, in combination, a wall of generally circular cross section defining a gas duct, the wall terminating at a propulsive jet outlet; a first pair of opposed valves rotatably mounted to swing oppositely about a first axis transverse to the duct; a second pair of opposed valves rotatably mounted to swing oppositely about a second axis transverse to the two aforementioned axes, the first and second axes being substantially coplanar and being upstream of the jet outlet; all the said valves being substantially in the form of lunes of a sphere of a size such as to substantially fit the wall when rotated to an open position maximizing the jet outlet area defined between the downstream edges of the four valves; means for moving the valves concurrently to reduce the area of the outlet area from the open area to a limit wherein opposed valves meet to close the jet outlet; and reverse thrust outlet means coupled to the valve moving means so as to be made effective upon closing the jet outlet.
 6. A jet propulsion nozzle comprising, in combination, a jet pipe having a jet outlet, the portion of the pipe adjacent to the outlet having an interior configuration substantially corresponding to a zone of a sphere; a first set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a first axis transverse to the outlet and diametral to the jet pipe spherical zone, the valves being of a diameter to provide a minimal radial gap between the jet wall and valves; a second set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a second axis transverse to the outlet and the first axis and intersecting the first axis, the valves of the second set being of a diameter to provide a minimal radial gap between the valves of the two sets; and means for rotating the valves about their respective axes concurrently between an open outlet position in which the valves are principally disposed within the said spherical zone and a closed outlet position in which the valves are substantially projected from the spherical zone.
 7. A jet propulsion nozzle comprising, in combination, a jet pipe having a jet outlet, the portion of the pipe adjacent to the outlet having an interior configuration substantially corresponding to a zone of a sphere; a first set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a first axis transverse to the outlet and diametral to the jet pipe spherical zone, the valves being of a diameter to provide a minimal radial gap between the jet wall and valves; a second set of opposed valves each substantially in the form of a spherical lune mounted for concurrent opposite rotation about a second axis transverse to the outlet and the first axis and intersecting the first axis, the valves of the second set being of a diameter to provide a minimal radial gap between the valves of the two sets; means for rotating the valves about their respective axes concurrently from an open outlet position in which the valves are principally disposed within the said spherical zone through a reduced area outlet position to a closed outlet position in which the valves are principally projected from the spherical zone and the downstream edges of at least one set of the valves meet to substantially block the jet outlet; and reverse thrust outlet means coupled to the valve rotating means so as to be made operative during travel from the reduced area position to the closed outlet position. 